1. Field of the Invention
The present invention generally relates to a ball bearing, and particularly to a ball bearing suitable for use in a spindle motor of an information equipment, an audio/video equipment, and especially of a magnetic or photomagnetic disk unit.
2. Description of the Related Art
A conventional ball bearing used in a spindle motor of a magnetic disk unit includes 7 to 10 balls as rollers disposed between inner and outer rings of the ball bearing. As for the material of the balls, high-carbon chromium bearing steel of HRC 63 to 67 in Rockwell hardness or Hv 772 to 900 in Vickers hardness has been used.
In a ball bearing having a seal or a seal plate on one of or each of opposite sides, generally, grease as lubricant is enclosed between inner and outer rings in order to prevent balls from being damaged in long-term high-speed rotations. The grease is enclosed between the inner and outer rings at the ratio of 10% to 15% of the volume of a space between the inner and outer rings.
Recently, with the development in improvement of a magnetic disk unit to make its capacity as well as recording density high, a spindle motor to be used in such a magnetic disk unit is required to be small in oscillation of Non-Repeatative-Runout (hereinafter abbreviated to xe2x80x9cNRROxe2x80x9d) and less in variation of the NRRO. Particularly, 70 nm or less NRRO is requested.
FIG. 5 shows a relationship between the number of balls of a ball bearing and the maximum value of NRRO when only one of the balls has a relative diameter difference of 60 nm while there is no relative diameter difference in the other balls. As shown in FIG. 5, it has been found that NRRO becomes smaller as the number of the balls is increased. Note that in the term xe2x80x9ca relative diameter differencexe2x80x9d means a difference in diameter between the balls.
Table 1 shows the maximum value of NRRO when the number of mountains in a swell (that is, integral harmonic undulation of the outer race track) and the number of balls are changed. Here, in the table, 0 means that the maximum value of NRRO is smaller than 1 nm.
Table 1
The maximum value of NRRO (nm) when the number of mountains in a swell (so called as an integral harmonic undulation) on an outer circumferential race track and the number of balls are changed
Note that in above-mentioned Table 1, MN defines a number of mountains in a swell and NB indicates a number or balls.
As shown in Table 1, by the recent investigation by the present inventors, it has been found that it is effective to make the number of balls larger than the number of balls used in a conventional ball bearing which is 8 or 10, and to set the number of balls to a number having many divisors, such as 12, 14, 16 or 18, in order to reduce the influence of the swell on the race track surface of the inner and outer rings. That is, when the number of balls is 12, 14, 16 or 20 the maximum value of NRRO becomes extremely small, even if the number of mountains of the swell) (that is, integral harmonic undulation of the outer race track) is varied.
On the other hand, when the number of balls is increased without changing the inner diameter of the inner ring and the outer diameter of the outer ring, the ball diameter must be also reduced in design. Then, the race track of the inner and outer rings approaches a pitch circle so that the volume of a space between the inner and outer rings is reduced. Therefore, there is a problem that the amount of grease for lubrication which can be charged into the space is reduced. In addition, there has been found another problem that, when grease of 10% to 15% of the volume of the space is enclosed between the inner and outer rings, NRRO increases because of the stirring resistance of the grease, and scattering of NRRO also increases.
It is therefore an object of the present invention to solve the foregoing problems.
It is another object of the invention to provide a ball bearing in which NRRO is reduced, and variation of NRRO is also reduced.
In order to achieve the above object, according to an aspect of the present invention, provided is a ball bearing having a plurality of balls disposed between inner and outer rings; wherein the number of the balls is made to be not smaller than 10, and lubricant the amount of which is not more than 8% of the volume of a space between the inner and outer rings is enclosed between the inner and outer rings.
According to the present invention, the number of balls is made to be not smaller than 10, and lubricant not more than 8% of the volume of a space between the inner and outer rings is enclosed between the inner and outer rings. Accordingly, it is possible to prevent increase of the stirring resistance of the lubricant such as grease even if the number of balls is increased. It is therefore possible to reduce NRRO, and reduce variations of NRRO.
When the present invention is applied to a spindle motor ball bearing in a disk unit, it is possible to attain low NRRO and in turn high surface density in the disk unit.
The additional experiment teaches that when the diameter difference between the balls is set at 30 nm or smaller, the NRRO can be satisfied under the condition that the number of balls is 10 or larger and an amount of filling grease is 10% or less.
Further, in the case of a requested product whose NRRO is 100 nm or less in maximum amplitude, setting of the filling grease amount at 10% or less will suffice even if the number of balls is 8 or larger. That is, the above-object can also be achieved by a ball bearing, according to the other aspect of the present invention, comprising: an inner ring; an outer ring; 8 or more balls disposed between inner and outer rings, a diameter difference of the balls being not larger than 30 nm; and a lubricant enclosed between the inner and outer rings, wherein an amount of the lubricant is not more than 10% of the volume of a space between the inner and outer rings.